A detailed analysis will be made of the cell kinetics of the transplantable rat mammary tumor 13762 grown in the ascitic phase and critical tissues (bone marrow and spleen) in untreated hosts and hosts treated with estrogen mustard. The cell kinetic analysis is based on the quantitative assessment of Feulgen-fluorescence stained 3HTdR autoradiographs by means of an automated microspectrophotometer. BIBLIOGRAPHIC REFERENCE: Neely, J.E. and Combs, J.W., Variation in the autoradiographic technique: I. Emulsion-developer combinations assessed by photometric measurement of single silver grains. J. Histochem. Soc. 24:1057-1064, 1976.